The overall goal of this program is to provide information on early phases of vertebrate CNS development. The proposed program will focus on two major issues: the development of midline structures that establish the pattern of axonal projections in the embryonic CNS and the mechanisms that control neural fate determination in brain. The aim of the first two subprojects will be to examine the early development of two midline structures that guide developing axons, the floor plate of the spinal cord and the optic chiasm of the visual system. In Project 1, Dr. Dodd will use PCR-based assays for floor plate specific genes to examine the control of floor plate induction by axial mesoderm. In Project 2, Dr. Mason will use neuroanatomical approaches to examine the specification of cell type in the developing optic chiasm. A second general aim will be the control of neural fate and differentiation in mammalian CNS. In Project 3, Dr. Jessell will use molecular approaches to examine the role of floor plate-specific genes, and proteins encoded by these novel genes, in neural differentiation. In Project 4, Dr. Hatten will use a novel in vitro system to examine the role of extrinsic signals in the specification of cerebellar granule cell identity. In Project 5, Dr. Heintz will use molecular biological approaches to identify novel genes that show regulated expression during cerebellar granule cell neurogenesis and differentiation.